Marshall JCM 900 fried! Need help :)

[sibo]

Again, thanks!
I'll look into the diode tomorrow but I did a quick recheck on R30 and it shows 38.5k again. What's a voltage dropper? Might this be part of the problem?

[g1]

Another quick item of interest is R27.
...
Previously we said:
R27: 150k (good, 150k accord. to schem g1 posted / should be 1.5k according to mine/2nd schem)
R27 is *clearly* 'smoked', and was either mis-measured, or read 'as expected' strictly by chance. Replace.

Good eye! Yes it should be replaced with 150K, even if it still reads 150K it has been overheated. Makes me worry about C11 a little.

Recheck R30 with one end disconnected. It should measure 22K, if not, replace it. The voltage dropper reduces the supply voltage that is going to the preamp tubes. I don't think it would cause your problem but it is possible.

[sibo]

Diode tests should be done in 'diode mode' on the meter (the symbol for a diode >| ), and you'll have to flip the leads (pos. and neg.) as you do it. If it's a good diode, it should read approx .7v when forward biased (pos. red-lead on anode, neg. black-lead on cathode [striped end]), and it should read 'infinite' when reverse bias

Ok so I did that and the digits shown on my display are 616 with right polarity and infinite the other way around. What does that mean and what exactly am I measuring?

[52 Bill]

Ok so I did that and the digits shown on my display are 616 with right polarity and infinite the other way around. What does that mean and what exactly am I measuring?

What that shows is that the diode is testing correctly.

By swapping the leads around, it shows that it conducts in one direction only, which is what makes it a diode.

The 616 display represents 0.616 volts dc. This is the voltage that it takes for the diode to start conducting or to turn on.

[sibo]

Here we go again, part two of my component-changing list. I will "over-do" it this time to minimize the risk of something else being bad. I found nothing on the +- tolerances, what should I aim for?
I would love some help on the missing data.

Power section:
R27: 150k, Effect?, Wirewound resistor with silicon coating, Axial connection
R30: 22k, 1W, Ceramic resistor, Axial connection
R35: 100Ohms, Effect?, Ceramic resistor, Axial connection
C11: 0,047uF, 250V (is this ac?), Connection type?
C12: 0,047uF, 250V (is this ac?), Connection type?
C15: Unknown

Pre section:
IC3: Does this need changing? I scraped some goo off it.
C3X: That big capacitor next to TR2, 33uF, 450V, Type?, Axial connection
C11: 0,1uF, 250V AC, Connection type?
C13: Capacitance?, 63V, Electrolytic, Axial connection
C18: 100uF, voltage?, Electrolytic, Axial connection
C31: Capacitance?, Voltage?, Type?, Radial connection
C32: Capacitance?, 35V, Radial connection
C45: 2,2uF, 63V, Electrolytic, Axial connection
R47: Unknown

If this doesn't help I guess I'll have to hack my axe through it...

[Audiotexan]

Here we go again, part two of my component-changing list. I will "over-do" it this time to minimize the risk of something else being bad. I found nothing on the +- tolerances, what should I aim for?

Shotgunning parts at it will only cost you money and time. So hold off just yet. But I feel your frustration. We've all been there. Just breath a moment, and it'll be alright! lol

I would love some help on the missing data.

Will try to help where we can!

As for the list above that you drew up...The only things that are needed thus far are:

Power section:
R27: 150k, Wirewound resistor

and if you did as g-one said here:

Recheck R30 with one end disconnected. It should measure 22K, if not, replace it.

As, while in circuit, it could measure differently simply due to other components affecting the reading.

So possibly adding R30 to the needed parts list:
R30: 22k, 1W, Ceramic resistor, Axial connection

Pre section:
IC3: Does this need changing? I scraped some goo off it.

No. Not unless it's been verified that it's bad.

If this doesn't help I guess I'll have to hack my axe through it...

Again, I feel your pain. Patience is a #$^$@ sometimes.

Once you get R27 changed out (which was already confirmed as needing replacement), and confirm verification that you used g-one's procedure, and possibly replacing R30 as well, then we'll have you get some voltage readings.

But we need to know where things stand once R27 has been replaced and R30 has been addressed/confirmed/replaced (if warranted) before we move to voltage readings. =)

[Audiotexan]

I would love some help on the missing data.

Again, this is just to cover your questions...but not needed unless mentioned in the previous post.

Power section:
R27: 150k, Effect?, Wirewound resistor (part of voltage divider network for bias)
R35: 100Ohms, Effect? (think of it as an extra fuse)
C11: 0,047uF, 250V (is this ac?), Connection type? (non-polarized coupling cap to pass signal, rating is DC)
C12: 0,047uF, 250V (is this ac?), Connection type? (non-polarized coupling cap to pass signal, rating is DC)
C15: Unknown (47nf -not sure what "Class X" is)

Pre section:
C3X: That big capacitor next to TR2, 33uF, 450V, Type?, Axial connection (standard electrolytic polarized PSU cap)
C11: 0,1uF, 250V AC, Connection type?
C13: Capacitance?, 63V, Electrolytic, Axial connection
C18: 100uF, voltage?, Electrolytic, Axial connection
C31: Capacitance?, Voltage?, Type?, Radial connection
C32: Capacitance?, 35V, Radial connection
C45: 2,2uF, 63V, Electrolytic, Axial connection
R47: Unknown

R47 pertains to the preamp circuit if I'm not mistaken, so I'll not make any further assumptions due to we've already made established there are revision differences and a preamp schem hasn't been posted and (more importantly confirmed) as matching your amp.

Basically though, most of your questions about things like radial vs axial, polarized vs non-polarized, are basics that need to be learned by reading some basic sites. Not meant to be rude/snarky in any way whatsoever, moreso that, some folks have taken a lot of time to do those types of wonderfully informative sites, and it'd be far easier than explaining it all here.

But the extreme basics are:
Radial: both leads on one end
Axial: one lead on each end

Polarized: has a side for +v, and a side for "ground" (can also be "less-positive" voltage: meaning not 'true' ground, but 'ground' with respect to the circuit, or what we refer to as a 'floating ground')
Non-polarized: typically used for passing AC signals (at least in the majority of our applications)

Ratings: WVDC (Working Voltage, DC)

Capacitor - Wikipedia, the free encyclopedia

Introduction to Capacitors, Capacitance and Charge

https://www.youtube.com/watch?v=CmII_BmOf0I

Hopefully that'll help get you off on the right foot

[g1]

Agree with Audiotexan about resisting the urge to shotgun replacing components not proven bad. For starters, you won't learn anything that way . But the bigger problem is you run the risk of introducing new problems which will interfere with the troubleshooting process.

"C15: Unknown (47nf -not sure what "Class X" is)" Class X & class Y caps are special safety rated caps that are approved to run "line to neutral" (class X) or "line to ground"-AKA "death cap" (class Y).
C15 is the bias feed cap, between the HV winding and the bias diode. I'm not sure if they are using class X here because of legal requirements, or because of how much damage can occur if that cap fails (loss of bias). In any case, it is quite a critical component.

[J M Fahey]

I guess they use it for the guaranteed reliability.
And even so they are using it beyond its rating: it´s meant for "up to" 250VAC lines, they are using it with some 350VAC .

[sibo]

Big thanks again. I took your advice and calmed down. And you're right, I do this because it's interesting. Otherwise I could've just turned it in at a tech shop and got it fixed! Also you're most definitely not being rude, Audiotexan. I have no prestige in this whatsoever, I'm just thankful for all the help you give me and I will start reading those links right now. Also, I will order R27 & R30 and after they're changed I will get back (I'll measure R30 with one end loosened first).

I'll keep you updated in a couple of days when the components have arrived and gotten changed <3

[sibo]

On a sidenote!
I actually have to admit smelling solder getting heated up turns me on I will be needing my own multimeter (the one I'm using is borrowed); would this be a good first invest? https://www.elfa.se/elfa3~se_en/elfa...%2C+AM-510-EUR

[Audiotexan]

...I will be needing my own multimeter (the one I'm using is borrowed); would this be a good first invest? https://www.elfa.se/elfa3~se_en/elfa...%2C+AM-510-EUR

At first I panicked, thinking holy crap if you're going to spend 379.00 might as well get a Fluke 87...then common sense kicked me in the teeth again, and I noticed the Zurich IP. (Thank you again Flagfox!)

At current conversion rates, I'm showing 1.00 SEK = 0.141336 USD, so that puts that meter around $54 USD. Modestly priced, and some nice features! True RMS, .1ohm accuracy, does small caps (up to 100uf) does frequency, and a count up to 3999. Not bad at all!

However, it all depends on how 'serious' (read as: how often) you'll be needing a DMM, as well as what you're working on. If you plan on doing a lot more in tube amps, I'd then suggest going '1 step up' and considering the 1000v model. Bear in mind, it's triple the price, but, considering that many tube amps operate with B+ in the range of 500+ volts, you're not leaving much room for 'disaster' to strike with the 600v model. [A guy I worked with cooked my 1000v Fluke 87 once on a TV flyback by not paying complete attn when he should have (hey, we all have off days!)] Regardless, if you're only going to be working on 'small' (25-50w) tube amps, and stuff around the house, then the meter you linked should be more than adequate.
Given you're already into a 100w amp that runs at 500v...

I look at it like this. IF you cook that meter even once, and replace it with the same, then you're already 2/3 of the way to the price of the 1000v model. The question lies with your outlook. Is it worth spending 3x as much for 50% more "insurance" that you won't even come close to cooking it (at least by raw B+)?

On a slightly different note, the single most important thing in a meter to me (aside from accuracy), is that it's True RMS.

A perk of the 1000v model I linked is that it's 10x more accurate in VDC readings, which is the majority of what you'll use. (Accuracy to within 100uv on the 1kv model, vs 1000uv accuracy on the 600v model) But again, that's a *perk*, the 1000uv reading is still quite accurate! Let your needs be your guide.

[Audiotexan]

"C15: Unknown (47nf -not sure what "Class X" is)" Class X & class Y caps are special safety rated caps that are approved to run "line to neutral" (class X) or "line to ground"-AKA "death cap" (class Y).

Ahhhhh! TY g-one!

[Audiotexan]

I guess they use it for the guaranteed reliability.
And even so they are using it beyond its rating: it´s meant for "up to" 250VAC lines, they are using it with some 350VAC .

Yeah, I was wondering about that as well...

I wish all manufacturers would put the damned voltages down.
/minor annoyance rant off

[Jazz P Bass]

Yeah, I ate too many $15.00 sense resistors on my Fluke 600 volters.

I have sinse gone to the 1000 volt model.